The present invention generally relates to solid state power controller technology and, more specifically, to devices and methods of switching in high power AC/DC solid state power controllers.
Solid State Power Controller (SSPC) technology is gaining acceptance as a modern alternative to the combination of conventional electromechanical relays and circuit breakers for commercial aircraft power distribution due to its high reliability, “soft” switching characteristics, fast response time, and ability to facilitate advanced load management and other aircraft functions.
While SSPCs with current rating under 15 A have been widely utilized in aircraft secondary distribution systems, power dissipation, voltage drop, and leakage current associated with solid state power switching devices pose challenges for using SSPCs in high voltage applications of aircraft primary distribution systems with higher current ratings.
A typical SSPC generally comprises a solid state switching device (SSSD), which performs the primary power on/off switching, and a processing engine, which is responsible for SSSD on/off control and a feeder wire protection.
Existing aircraft applications employ exclusively a metal oxide semiconductor field effect transistor (MOSFET) as a basic solid state component for building up the SSSD. It features easy control, bi-directional conduction characteristic, and resistive conduction nature with positive temperature coefficient. To increase the current carrying capability and reduce the voltage drop or power dissipation, the SSSD comprises multiple MOSFETs generally connected in parallel. However, this set up does not warrant an increased capability to handle higher fault current. During SSSD turn-off transients, generally, neither all the MOSFETs turn off simultaneously nor the fault current distributes evenly among the MOSFETs in such a short time. As a result, fault current capability of single MOSFET has to be considered as the worst case scenario in the design of SSSDs. Meanwhile, the resistance and, therefore, power dissipation of the MOSFET turned on increase significantly with its voltage ratings. That increase greatly limits the MOSFET potential applications in the high voltage environments, such as 115VAC, 230VAC, 270VDC, and 540VDC, etc., in the aircraft.
Similar to the MOSFET in gate controls, an insulated gate bipolar transistor (IGBT) features high current carrying capability, low conduction loss at high current, availability of high voltage ratings, etc. However, a greater than 1.7V voltage associated with IGBT on-state is still considered too high and would introduce errors at the voltage zero crossing detection. Furthermore, the limited reverse blocking capability makes use of the conventional IGBT difficult for AC applications and a diode would have to be added, further impacting the on state voltage. A newly developed reverse blocking IGBT (RB-IGBT) is designed for bi-directional power switching. But the inherent “dead band” associated with a greater than 2V on-state voltage of RB-IGBT results in noticeable distortions in the controlled current that are highly undesirable, if not unacceptable to existing Aerospace Electromagnetic Interference and Power Quality requirements, for power distribution applications.
As can be seen, there is a need for to provide a practical solution for the solid state power switch to be used in high power AC/DC SSPCs (either with higher current ratings, e.g. >15 A, or in higher voltage applications, e.g. ≧115VAC), particularly using existing commercially available semiconductors. There is also a need to provide such a solution, which will result in reduced power dissipation, improved reliability and fault current handling capability, and no current distortions.